The invention relates to a fluid friction clutch having a housing and having a clutch plate which is arranged so as to be rotatable relative to the housing and is rotationally fixedly arranged on an end of a shaft. The shaft is mounted centrally within the housing and supports, at its other end, an active element which is to be driven by the clutch. The clutch has a working chamber which can be pressurized with clutch fluid and is formed between the housing and the clutch plate. The clutch also has a reservoir chamber, which is formed in the housing, for the clutch fluid. A supply duct leads from the reservoir chamber to the working chamber. A back-pumping system returns the clutch fluid from the working chamber to the reservoir chamber. A valve arrangement positioned in the supply duct controls the clutch fluid which is supplied to the working chamber.
In a clutch of said type, the transmission of torque is dependent on the quantity of clutch fluid situated in the working chamber. If a small amount of fluid is situated in the working chamber, then there is a comparatively large degree of slip between the primary and the secondary side, which slip decreases with increasing filling of the working chamber. The rotational speed of the active element changes correspondingly. In order to ensure a constant rotational speed, that is to say a stable operating point, of the secondary side, the relative volume flow rates of the clutch fluid into and out of the working chamber should be equal regardless of the rotational speed of the primary side. The design of the pump system for the clutch fluid therefore has a decisive influence on the operational stability of the clutch.
In the case of a clutch of the type described above and shown in U.S. Pat. No. 6,026,943, a scraping element is arranged in the working chamber between the primary side and the secondary side, which scraping element is part of a dynamic-pressure pump. The supply flow of clutch fluid into the working chamber is controlled by means of a valve arranged in the supply line. Once clutch fluid passes into the working chamber, the secondary side is driven and the relative speed between the primary and secondary sides is reduced. This has the result that the dynamic pressure at the scraping element falls and the effectiveness of the pump is reduced. A feedback of external influences on the secondary-side active element can now lead, depending on the type of influence, to the rotational speed of the secondary side increasing or decreasing with unchanged rotational speed of the primary side. The relative speed and therefore the dynamic pressure at the scraping element is correspondingly varied. In this known clutch, this leads to a drift away from the operating point, and the transmission of torque is unstable.
It is an object of the present invention to provide a fluid friction clutch of the type specified above, but in which a predefined nominal rotational speed can be maintained in a stable fashion under all circumstances.